After completing a law degree at Yale University, he was admitted to the bar.
He lost a re-election bid in 1938 to J. Hector Paquin during a Republican sweep of state offices.
In 1940 he was a substitute delegate to the Democratic National Convention, where Franklin D. Roosevelt was nominated for the third time for president.
In 1940, Cappelli was elected Lieutenant Governor of Rhode Island alongside J. Howard McGrath.
On April 12, 1944, he was appointed an associate justice of the Rhode Island Superior Court.